Comb polymers are used as dispersants to increase the stability of colloidal suspensions containing multivalent or high concentrations of monovalent ions. Stabilized colloidal suspensions and methods of forming stabilized colloidal suspensions are described, including suspensions containing ceramic precursors or bioactive agents useful in forming ceramic substrates or pharmaceutical compositions, respectively.

wherein said suspension comprises said monovalent ions at a concentration of at least 0.1 M,

the particles have an average effective diameter of from 1 nanometer to 3 microns,

the comb polymer comprises a polymer having ionizable and non-ionizable side chains, and

a ratio of the ionizable to non-ionizable side chains is from 20:1 to 1:1.

33. In a colloidal suspension including ceramic precursor particles, multivalent ions at a concentration of at least 0.001 M, a carrier liquid, and a dispersant, the improvement comprising at least one comb polymer as the dispersant,

wherein the particles have an average effective diameter from 1 nanometer to 3 microns.

the comb polymer comprises a polymer having ionizable and nonionizable side-chains, and

a ratio of the ionizable to the nonionizable side-chains is from 20:1 to 1:1.

34. In a colloidal suspension including ceramic precursor particles having an average effective diameter of from 1 nanometer to 3 microns, monovalent ions at a concentration of at least 0.1 M, a carrier liquid, and a dispersant, the improvement comprising at least one comb polymer as the dispersant, wherein the comb polymer comprises a polymer having ionizable and non-ionizable side chains in a ratio of ionizable to non-ionizable side chains of from 20:1 to 1:1.

35. The colloidal suspension of claim 1,

wherein said suspension is a pharmaceutical composition,

said particles comprise a bioactive agent, and

said comb polymer comprises ionizable and nonionizable side-chains.

36. The pharmaceutical composition of claim 35, wherein said comb polymer is water-soluble.

37. The pharmaceutical composition of claim 35, wherein said comb polymer is bio-compatible.

38. The colloidal suspension of claim 16,

wherein said suspension is a pharmaceutical composition,

said particles comprise a bioactive agent, and

said comb polymer comprises ionizable and nonionizable side-chains.

39. The pharmaceutical composition of claim 38, wherein said comb polymer is water-soluble.

40. The pharmaceutical composition of claim 38, wherein said comb polymer is bio-compatible.

Description

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/426,297, filed Nov. 14, 2002, entitled “Controlled Dispersion of Colloidal Suspensions by Comb Polymers,” which is incorporated by reference.

BACKGROUND

Most any liquid that contains particles that are not fully solubilized can be characterized as a colloidal suspension. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug discovery. Colloid based products include paints, inks, coatings, ceramic precursors, cosmetics, and pharmaceutical compositions. In the case of ceramics, concentrated colloidal suspensions may be fabricated into dense components by sintering.

The viscosity of colloid suspensions can vary over a wide range from free-flowing liquid to flocculated gel. Dispersants that modify viscosity are often added to colloidal suspensions. A major benefit of viscosity control is the ability to lower the viscosity of a concentrated suspension. By lowering the viscosity, a suspension may be processed through pumps, pipes, and other machinery in a simpler and more cost effective manner. By tailoring interactions between colloidal particles through the addition of a dispersant, one can alter the viscosity of colloids to make them suitable for use in a broad array of applications. Through dispersant addition, colloidal suspensions may be processed at higher solids content than would otherwise be possible.

Polyacrylic acid (PAA) is the polyelectrolyte dispersant most widely used for the aqueous processing of ceramics. PAA contains carboxylic acid groups, one per monomer unit, along its backbone. By adding PAA, the dispersion of the colloidal particles is increased, thus reducing aggregation or flocculation. As flocculation is reduced, viscosity decreases. Another common dispersant used in ceramics processing is poly(methacrylic acid) (PMAA). PMAA contains carboxylic acid groups and methyl substituents on the backbone.

These polyelectrolyte dispersants are believed to reduce flocculation by stabilizing the colloid particles through the negative charge generated when the carboxylic acid functional groups are deprotonated. When deprotonated or ionized, the negatively charged carboxylic groups are believed to form an electrostatic repulsive barrier between the particles that form the colloid, thus reducing flocculation. It has also been postulated that the steric requirements of the dispersant provides additional stabilization.

PAA dispersant systems become less effective at reducing viscosity when higher ionic strength colloidal suspensions, especially those containing multivalent ions, are involved. It is believed that the multivalent ions interfere with the electrostatic repulsive barrier of the carboxylic groups. This may result in a decreased electrostatic repulsive barrier between the colloidal particles, thereby increasing flocculation. Hence, there is a need for dispersants that can reduce flocculation of suspended particles and thus provide lowered viscosity in high ionic strength colloidal suspensions.

BRIEF SUMMARY

In one aspect, a colloidal suspension includes particles, such as metals, ceramic precursors, semiconductors, polymers, biodegradable polymers, bioactive agents, and mixtures thereof; a comb polymer; and multivalent ions having a suspension concentration of at least 0.001 M.

In another aspect, a colloidal suspension includes particles, such as metals, ceramic precursors, semiconductors, polymers, biodegradable polymers, bioactive agents, and mixtures thereof; a comb polymer; and monovalent ions having a suspension concentration of at least 0.1 M.

In another aspect, a method of forming a mixture is provided where the mixture includes a comb polymer; a carrier liquid; particles, such as metals, ceramic precursors, semiconductors, polymers, biodegradable polymers, bioactive agents, and mixtures thereof; and multivalent ions having a suspension concentration of at least 0.001 M.

In another aspect, a method of forming a mixture is provided where the mixture includes a comb polymer; a carrier liquid; particles, such as metals, ceramic precursors, semiconductors, polymers, biodegradable polymers, bioactive agents, and mixtures thereof; and monovalent ions having a suspension concentration of at least 0.1 M.

In another aspect, a method of forming a colloidal suspension is provided where a comb polymer is mixed with a mixture that includes a carrier liquid; hydroxyapatite particles, titanium oxide particles, lead zirconate particles, titanate particles, alumina particles, silica particles, zirconia particles, silicon nitride particles, barium titanate particles, or silicon carbide particles; or mixtures of these particles; and multivalent ions having a suspension concentration of at least 0.001 M.

In another aspect, a method of forming a colloidal suspension is provided where a comb polymer is mixed with a mixture that includes a carrier liquid; hydroxyapatite particles, titanium oxide particles, lead zirconate particles, titanate particles, alumina particles, silica particles, zirconia particles, silicon nitride particles, barium titanate particles, silicon or carbide particles; or mixtures of these particles; and monovalent ions having a suspension concentration of at least 0.1 M.

In another aspect, a method of making a ceramic substrate is provided that includes solidifying a colloidal suspension to form a substrate where the colloidal suspension includes a carrier liquid, ceramic precursor particles, a comb polymer, and multivalent ions having a suspension concentration of at least 0.001 M.

In another aspect, a method of making a ceramic substrate is provided that includes solidifying a colloidal suspension to form a substrate where the colloidal suspension includes a carrier liquid, ceramic precursor particles, a comb polymer, and monovalent ions having a suspension concentration of at least 0.1 M.

In another aspect, a method of lowering the viscosity of a colloidal suspension includes adding a comb polymer to a mixture that includes multivalent ions having a suspension concentration of at least 0.001 M.

In another aspect, a method of lowering the viscosity of a colloidal suspension includes adding a comb polymer to a mixture that includes monovalent ions having a suspension concentration of at least 0.1 M.

The scope of the present invention is defined solely by the appended claims, and is not affected to any degree by the statements within this summary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative example of a comb polymer.

FIG. 2 shows an illustrative example of forming a comb polymer having nonionizable and ionizable side-chains provided by different monomers.

FIG. 5 is a plot showing the significantly reduced viscosity of a colloidal suspension when PAA-PEO versus PAA dispersant is used in the presence of multivalent cations.

FIG. 6 is a plot showing the significantly reduced viscosity of a colloidal suspension when PAA-PEO versus PAA dispersant is used in the presence of a high concentration of monovalent cations.

FIG. 7 is a plot of apparent viscosity as a function of shear rate for BaTiO3 suspensions with varying PAA concentration.

FIG. 8 is a plot of apparent viscosity as a function of shear rate for BaTiO3 suspensions with varying PAA-PEO(1 K) concentration.

FIG. 9 is a plot of apparent viscosity as a function of shear rate for BaTiO3 suspensions with varying PAA-PEO(2K) concentration.

DETAILED DESCRIPTION

The present invention includes suspensions stabilized by comb polymer dispersants and methods of using comb polymer dispersants to regulate the stability of colloidal suspensions having a high ionic strength, including suspensions containing multivalent ions. In relation to polymers having only ionizable side-chains, the comb polymers of the present invention markedly reduce the viscosity of high ionic strength suspensions. By modifying the structure of the comb polymer, its concentration in the colloidal suspension, and the properties of the carrier liquid, the viscosity of the suspension may be altered by several orders of magnitude.

Colloidal Suspensions

Colloidal particles have a substantial fraction of their atoms or molecules at the surface. When placed in a carrier liquid, an interface exists between the surface of the particles and the carrier liquid. The behavior of the resultant colloid, including stability, digestibility, film forming properties, and viscous and elastic properties, is chiefly determined by how this surrounding interface interacts with the surface of the colloidal particles and the carrier liquid.

Solutions, unlike colloidal dispersions or suspensions, lack an identifiable interface between their solubilized molecules and the solvent. In solutions, the solubilized molecules are in direct contact with the solvent, while in colloidal suspensions only the surface of the microparticles are in direct contact with the carrier liquid. Hence, the carrier liquid does not solubilize the particles that make up a colloid; instead, the carrier liquid “carries” the particles. By carrying the particles, a suspension or dispersion results. The terms suspension and dispersion are used interchangeably.

The interfaces between the suspended colloidal particles, and the carrier liquid or liquid mixture in which they reside, play the dominant role in determining the behavior and capabilities of the colloidal dispersion. Colloidal suspensions are considered stable if the particles that form the colloid are separated or deflocculated, i.e., not aggregated or flocculated. In relation to colloidal suspensions, stability is the ability of the suspension to resist change, such as aggregation, over time.

Long-range attractive forces, such as van der Waals forces, are believed to pull colloidal particles together. When colloidal particles are pulled together, the colloidal dispersion or suspension is destabilized. This destabilization is often referred to as aggregation or flocculation and can result in precipitation of the aggregated particles from the colloidal suspension. Additionally, as the suspension flocculates its viscosity increases.

As colloidal suspensions flocculate, they can go from a liquid phase to a gel phase. Thus, as suspension stability is reduced, the liquid becomes a gel. There is no specific point at which a liquid becomes a gel. In general, however, liquids freely flow while gels do not. A liquid will conform to the shape of a container in which it is placed, while a gel can have a physical form separate from the container where it resides.

Alternatively, columbic, steric, and other repulsive interactions are believed to repel colloidal particles from each other. If the particles cannot aggregate together, the stability of the colloidal suspension is increased and flocculation may be reduced. Thus, through the reduction of flocculation, a suspension's viscosity may be decreased.

In the past it was believed that comb polymers having nonionizable side-chains, in addition to ionizable side-chains, provided no practical benefit over commonly used dispersants having only ionizable side-chains, such as polyacrylic and polymethacrylic acid, in the stabilization of colloidal suspensions. It was also believed that varying the length (molecular weight) of the nonionizable side-chains had no appreciable effect on the ability of the comb polymer to stabilize colloidal suspensions. This position is supported in Bergstrom et al., The effect of anionic polyelectrolytes on the properties of aqueous silicon nitride suspensions, Journal of the European Ceramic Society 20 (2000) 431–440.

Surprisingly, the present invention provides embodiments that can significantly reduce the tendency of colloidal particles to flocculate in relation to dispersants having only ionizable side-chains through the use of comb polymer dispersants having nonionizable, in addition to ionizable, side-chains. The results are especially beneficial when the colloidal suspensions have high ionic strength arising from multivalent ions, and/or high concentrations of monovalent ions.

While not wishing to be bound by any particular theory, it is believed that the nonionizable side-chains of the comb polymers “shield” the ionized side-chains from ion bridging interactions (where an ion attracts dispersant coated particles), especially from multivalent ions. The nonionizable side-chains are also believed to impart steric stabilization over an interparticle separation distance that increases with the molecular weight of the nonionizable side-chains, thus making the suspension less sensitive to changes in ionic strength. In this manner, the comb polymers are believed to maintain repulsive forces between the suspended particles, even in the presence of the multivalent ions.

One possible explanation regarding how the comb polymers increase the repulsive forces between the suspended particles is that the like-charged comb polymers attach to the particles and congregate about the ions, thus forming a “protective shield.” The repulsive forces generated by the adsorbed comb polymers are believed to reduce the tendency of the particles to aggregate, thus counteracting the attractive van der Waals forces. The charge carried by the comb polymers may be positive or negative; however, the net charge of the comb polymers adsorbed on the colloidal particle surfaces may be positive, negative, or neutral.

Carrier liquid

A feature of the present approach to colloidal viscosity control is that comb polymer dispersants are used to modify the suspension. Another consideration is the polarity of the carrier liquid in which the particles are suspended. The colloidal dispersions of the present embodiments contain particles that are not solubilized; instead, the particles are suspended in a carrier liquid. Depending on the comb polymer dispersant used, varying degrees of carrier liquid polarity may be used to further tune suspension viscosity. Mixtures of polar liquids and less-polar, or even non-polar liquids can be used to fine tune the polarity of the liquid carrier.

While many polar carrier liquids may be used to form the colloid dispersions, water is especially preferred at present. Other presently preferred carrier liquids that are less polar than water include alcohols, such as methanol, propanol, ethanol, and t-butanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, acetonitrile, acetic acid, hexamethylphosphoric triamide (HMPA), tetrahydrofuran (THF), N,N-dimethylacetamide, N-methyl-2-pyrrolidone, tetramethyl urea, glycerol, ethylene glycol, or mixtures thereof.

Stabilized Against Flocculation

The stability of a colloidal suspension is defined as its ability to resist flocculation. The greater the number of particles that can be added to a given volume of carrier liquid without flocculation, the greater the stability of the suspension.

A colloidal suspension is stabilized against flocculation when at least 90% of the particles can be observed as being individual, rather than aggregated in groups of two or more. This determination is made by diluting a sample of the colloidal suspension to 1 part-per-million solids, placing the sample on a slide, and observing the sample by light microscopy.

Particles

The particles are suspended or dispersed in a carrier liquid to form a colloidal suspension. Preferable particles include metals, ceramic precursors, semiconductors, polymers, biodegradable polymers, bioactive agents, proteins, liposomes, and other biomolecules, or mixtures thereof. Preferably, the particles do not include calcium silicate hydrates, cement, cement precursors, calcium sulfate hemihydrates, gypsum, and gypsum precursors.

More preferable metal particles are selected from the group consisting of tool steels, molybdenum, nickel, gold, silver, platinum, titanium-aluminum-vanadium alloys, tungsten, and aluminum, or mixtures or alloys thereof. Especially preferable metal particles are selected from the group consisting of tool steels, molybdenum, and nickel, or mixtures thereof.

More preferable polymer particles are selected from the group consisting of acrylic latexes, poly(ethyl methacrylate), cellulose polystyrene, poly(methyl methacrylate), poly(lactic acids), natural rubber, polyethylene, and poly(vinyl chloride), or mixtures thereof. Especially preferable polymer particles are selected from the group consisting of acrylic latexes, poly(ethyl methacrylate), and cellulose, or mixtures thereof.

Preferable particles may also be bioactive agents. Many bioactive agents, including proteins, form multivalent ions. Thus, the current embodiments can stabilize suspensions containing multivalent and monovalent bioactive agents in addition to inorganic ions or counterions.

Other bioactive agents, which may be delivered by colloidal suspensions, include chemotherapeutic agents, such as carboplatin, cisplatin, paclitaxel, BCNU, vincristine, camptothecin, etopside, cytokines, ribozymes, interferons, oligonucleotides and oligonucleotide sequences that inhibit translation or transcription of tumor genes, functional derivatives of the foregoing, and generally known chemotherapeutic agents such as those described in U.S. Pat. No. 5,651,986.

Not only can many of these bioactive agents, including proteins, directly serve as the particles in a colloidal suspension, but they can also be mixed with biodegradable compositions or polymers to form particles. By grinding a mixture containing one or more biodegradable composition and bioactive agent into particles, colloidal suspensions may be formed. Many useful biodegradable compositions suitable for use with bioactive agents may be found in U.S. Pat. No. 5,416,071.

Depending on their surface structure and the nature of the carrier liquid, the average effective diameter of particles suitable for colloid formation can vary over a wide range. By “average effective diameter” it is meant the longest dimension of the particle. Thus, if a particle is 0.01 μm in one dimension and 10 μm in another, the effective diameter of the particle is 10 μm.

Preferred particles have average effective diameters of 1 nanometer to 100 microns, more preferably 10 nanometers to 50 microns, and most preferably 20 nanometers to 3 microns.

Comb Polymer

Monomer units are the individual moieties that are repeated to form polymers. Multiple monomer units are covalently attached when in the form of a backbone of a polymer. Polymers that are made from at least two different monomer units are referred to as copolymers. Polymerizing or copolymerizing describes the process by which multiple monomers (i.e. chemical compounds) are reacted to form covalently linked monomer units that form polymers or copolymers, respectively. A discussion of polymers, monomer units, and the monomers from which they are made may be found in Stevens, Polymer Chemistry: An Introduction, 3rd ed., Oxford University Press, 1999.

Comb polymers are polymers that have backbones to which side-chains are attached. A discussion of comb polymers and their various structures may be found in D. H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic Press, 1983 (pp. 28–30). An illustrative example of a comb polymer is depicted in FIG. 1.

The comb polymers of the current invention contain at least two types of side-chains and preferably are water-soluble. By water-soluble it is meant that at least one gram of the polymer may be solubilized in one Liter of the carrier liquid. Preferably, the first type of side-chain, referred to as ionizable, has moieties that ionize at the pH of the colloidal suspension. These moieties can ionize to either cationic or anionic states when they dissociate in the carrier liquid. If desired, comb polymers that form both cationic and anionic moieties upon dissociation may be combined to alter the stability of the colloidal suspension. Preferably, the second type of side-chain, referred to as nonionizable, does not ionize at the pH of the colloidal suspension.

Although many variations to synthesize comb polymers are known, there are two basic pathways. The first is to form a copolymer from a monomer or monomer unit having ionizable side-chains and a monomer or monomer unit having nonionizable side-chains. An illustrative example of this synthesis pathway is shown in FIG. 2.

By varying the ratio of ionizable (m) to nonionizable (n) monomers combined to form the polymer, the resultant comb polymer may be tuned to stabilize a specific colloidal suspension, or group of suspensions. By maintaining a sufficient number of ionizable (hydrophilic) side-chains, in relation to nonionizable side-chains, the water solubility of the comb polymer may be maintained.

The second general comb polymer synthesis method begins with a polymer having ionizable groups on the backbone that can be functionalized with nonionizable groups (cap). By capping a portion of the polymer's ionizable groups with nonionizable groups, a comb polymer having both ionizable and nonionizable side-chains may be formed. An illustrative example of this synthesis pathway is shown in FIG. 3.

By controlling the reaction of the capping agent with the polymer, the number of ionizable side-chains that are converted to capped nonionizable side-chains may be varied. Thus, the resultant comb polymer may be tuned to stabilize a specific colloidal suspension, or group of suspensions. As above, water-solubility can be maintained by limiting the degree of capping of the hydrophilic backbone.

More preferable bio-compatible polymer backbones are selected from the group consisting of polyamines, polyamides, polyesteramides, poly(lactic acid), poly(glycolic acid), nucleic acids (DNA and RNA), proteins, polysaccharides, and poly(caprolactone), or mixtures thereof. Especially preferable bio-compatible polymer backbones are selected from the group consisting of polyamines, polyamides, and polyesteramides, or mixtures thereof.

Especially preferred are comb polymers made from the reaction of a hydrophilic polyacrylic acid backbone with polyethylene oxide caps that results in a comb polymer having ionizable side-chains incorporating a carboxylic acid moiety and nonionizable side-chains incorporating a polyethylene oxide moiety. Referring to FIG. 4, polyacrylic acid is capped with polyethylene oxide. By varying m, n, and/or o the molecular weight of the comb polymer and the ratio of ionizable versus nonionizable side-chains may be altered. By varying p, the molecular weight of the cap may be altered. As used in this specification and appended claims, the term “a comb polymer” includes one or more polymeric units and one or more types of polymeric units.

In addition to changing the atomic structure of the backbone and side-chains, and the ratio between ionizable and nonionizable side-chains (m versus n), the molecular weight of the backbone (determined by o) and side-chains (determined by p) may be varied to control suspension stability. Preferable ratios of ionizable to nonionizable side-chains (m versus n) are from 20:1 to 1:1 and more preferably from 10:1 to 2:1. Preferable average molecular weights (grams/mole) of the comb polymer backbone (o) are from 1,000 to 15,000 and more preferably from 2,000 to 10,000. When polyacrylic acid backbones are used, preferable values for p result in polyethylene oxide caps having average molecular weights (grams/mole) from 100 to 5,000, more preferably from 600 to 3,000, and most preferably from 900 to 2,200.

High Ionic Strength Suspensions

The present invention is especially useful to stabilize high ionic strength colloidal suspensions. High ionic strength suspensions typically result from two sets of circumstances.

The first are colloidal suspensions that contain high concentrations of monovalent ions solvated in the carrier liquid. Monovalent ions are those that attain a +1 or −1 ionization state when ionized in the carrier liquid. High concentration is preferably defined as a molarity of 0.1 or greater. More preferably the molarity is 0.5 or greater, and most preferably the molarity is 1.0 or greater.

A second circumstance resulting in high ionic strength involves colloidal suspensions that contain multivalent ions solvated at a molarity of 0.001 or greater in the carrier liquid. Preferably, the suspension contains multivalent ions at a molarity of 0.01 or greater, more preferably the suspension contains multivalent ions at a molarity of 0.1 or greater. Multivalent ions are those that attain a +2 or higher, or a −2 or lower, ionization state when ionized (solvated) in the carrier liquid.

The ionization state of an ion is determined by the charge it adopts in the carrier liquid. Depending on the atomic make-up of the ion, ions may be present in the +1, −1, +2, −2 or higher and lower ionization states (+3 or +4, for example).

Preferred monovalent cations include, but are not limited to ammonium; metals, such as Cu+, K+, Li+, and Na+; alkyl metal complexes, such as CH3Hg+; metal oxides, such as NpO2+, PaO2+, and PuO2+; metal hydroxides, such as CaOH+, FeOH+, BaOH+, AI(OH)2+, Ir(OH)2+, Pb(OH)3+, and Nb(OH)4+; cationic metal halides, such as AI(X)2+, CaX+, Cr(X)2+, and Pb(X)3+, where X is fluoride, chloride, bromide, or iodide; and mixtures thereof.

Preferred monovalent anions include, but are not limited to halides; alkoxides; hydroxides; deprotonated acids, such as CN−, SCN−, OCN−, NO3−, NO2−, H2PO4−, and HSO4−; and anionic metal halides, such a Al(X)4−, Ba(X)3−, and Fe(X)3−, where X is fluoride, chloride, bromide, or iodide; and mixtures thereof.

Preferred multivalent anions include, but are not limited to elements and multi-atom complexes capable of carrying a 2−or lower charge, such as Te2−, S−, HAsO42−, MnO42−, Ca(OH)42−, Mg(OH)42−, Hg(OH)42−, V2O74−, and Mn(OH)42−; halide complexes capable of carrying a −2 or lower charge, such as Co(X)42−, Cu(X)42−, Fe(X)42−, Th(X)62−, and Ti(X)62−, where X is fluoride, chloride, bromide, or iodide; and mixtures thereof.

The molarity of ions solvated in a colloidal suspension is determined by analyzing a sample of the carrier liquid, which contains the ions, by Inductively Coupled Plasma (ICP) analysis. The carrier liquid may be initially separated from the suspended particles, by for example, centrifugation. By this method, the quantity and atomic identity of the solvated ions is determined. Thus, the molarity of ions in the carrier liquid may be determined and their oxidation states may be implied from their identities.

High ionic strength colloidal suspensions may be initially created (such as through the addition of electrolytes that dissociate to form ions in the carrier liquid), or may form over time through dissolution of the suspended particles of an initially lower ionic strength suspension. One way that the ionic strength of a suspension increases over time is when some of the suspended colloidal particles, or the ions on their surface, are solvated into the carrier liquid. This increased solvation over time, or “dissolution,” can increase the ionic strength of a colloidal suspension by increasing the solvated ion concentration in the suspension. Colloidal suspensions containing small particles, such as particles having effective diameters in the nanometer range, are especially susceptible to dissolution. By stabilizing colloidal suspensions that increase in ionic strength over time, the preferred embodiments beneficially increase the long term stability, or shelf life, of the suspension.

Determination of Critical Concentration

To achieve the desired stabilization of a colloidal suspension containing ceramic precursor, bioactive agents, or other particles, a critical concentration may be determined. By adding increasing amounts of comb polymers with ionizable and nonionizable side-chains to a suspension and measuring the viscosity, one can determine the optimal (lowest) amount of a specific comb polymer required to attain the desired viscosity reduction (stability). Critical concentration is defined as the lowest amount of comb polymer required to attain stability for a colloidal suspension.

As shown in the viscosity plots of FIGS. 8 and 9, for a colloidal suspension containing BaTiO3 particles, the critical concentration of PAA-PEO is at the 1.2 mg level per m2 of BaTiO3 surface. Neither greater (2.0 mg), nor lesser (0.6 mg), amounts of the comb polymer dispersant lower viscosity as well.

The present embodiments are especially useful in the area of ceramics fabrication where it is desirable to produce ceramic precursor colloidal suspensions containing multivalent ions and/or high concentrations of monovalent ions. As the concentration of particles is increased in the suspension, their tendency to aggregate or flocculate increases. The use of effective dispersants to reduce viscosity allows for preparation of high solids-content ceramic precursor suspensions.

If appropriate ceramic precursor particles are chosen, the resultant colloidal suspension may be solidified to directly form solid structures, such as ceramic substrates for electronic devices, or to form electronic materials, such as would be suitable for use in capacitors. The colloidal suspensions may be solidified using heat (sintering or firing in a kiln, for example) or other methods which bring about the desired solidification. In the case of some devices, only partial solidification may be desired. Depending on the device or substrate desired, the particles may be allowed to partially flocculate, thus becoming gel-like in nature, or even precipitate, prior to solidification.

Many pharmaceutical uses also exist for a colloidal suspension having adjustable viscosity. In these applications, bio-compatible comb polymers, such as those with polylactic acid backbones, are preferred. Bio-compatible means the comb polymer is not toxic to living systems at the quantity used. Because the amount of colloidal stabilization provided by a specific type and quantity of bio-compatible comb polymer is pH dependent, pH changes that occur when the colloidal suspension is administered orally, subcutaneously, or intravenously can be used to alter the phase of the comb polymer stabilized colloid. One such use, for example, is to alter the viscosity of injectable pharmaceutical compositions containing one or more bioactive agents.

It can be advantageous to have a very low viscosity drug composition that can pass through a very fine needle into the body, but yet have the drug composition stay localized in the tissue at the region of injection. This localized region is characterized by its phase separation from the physiological fluid and its decreased fluidity relative to the original suspension. By adding a comb polymer dispersant to the colloidal drug composition that lowers viscosity at the pH of the delivery suspension, but not at physiological pH, a drug composition can exist at a relatively low viscosity in the syringe, but at a relatively higher viscosity in the body. In this fashion, the pharmaceutical is easily delivered to a specific location.

Barium titanate (BaTiO3) nanoparticles (BT-16 K-Plus, Cabot Co., Boyertown, Pa.) with an average effective diameter of 60 nm, a specific surface area of 16.4 m2/g, and a density of 5.88 g/cm3 served as the ceramic precursor powder in this study.

BaTiO3 suspensions were prepared by dispersing an appropriate amount of this powder in an aqueous dispersant solution. The solution was premixed by adding an appropriate dispersant amount to deionized water followed by magnetic stirring for 0.5 h. Upon adding BaTiO3 powder, the suspension was ultrasonicated (F550 Sonic Dismembrator, Fisher Scientific, Kosmonautu 324, CZ-53009 Pardubice, CZECH REPUBLIC) for 5 min using a 1 s on/off pulse sequence. The suspension pH was adjusted to 9 using 0.1 M HNO3 and 0.1 M NH4OH solutions. The suspensions were then magnetically stirred for 24 h.

As can be seen from FIG. 5, the BaTiO3-PAA based system exhibited large viscosities upon dosing with only 0.001 M BaCl2 salt, whereas the BaTiO3-PAA-PEO comb-polymer based system retained low viscosities upon dosing with 0.001 M BaCl2 salt.

Example 2

Preparation of a Comb Polymer Stabilized Colloidal Suspension Containing a High Concentration of Monovalent Ions.

As can be seen from FIG. 6 the BaTiO3-PAA system exhibited large viscosities upon dosing with 0.003 M KCI salt, whereas the BaTiO3PAA-PEO comb-polymer based system retained low viscosities upon dosing with 0.003 M KCl salt.

Prophetic Example 1

Preparation of a Pharmaceutical Composition Stabilized with a Bio-Compatible Comb Polymer.

A pharmaceutical composition including a chemotherapeutic drug and a biodegradable polymer is prepared and formed into particles. Suspensions of the particles are prepared by dispersing the particles in an aqueous solution of comb polymer dispersant.

The comb polymer dispersant consists of a poly(lactic acid) backbone with PEO caps.

An aqueous solution of the dispersant is premixed by adding the dispersant to deionized water followed by magnetic stirring for 0.5 h. The pharmaceutical composition particles are added to the aqueous dispersant solution. Upon adding the pharmaceutical particles, the suspension is ultrasonicated (F550 Sonic Dismembrator, Fisher Scientific, Kosmonautu 324, CZ-53009 Pardubice, CZECH REPUBLIC) for 5 min using a 1 s on/off pulse sequence. The suspension pH is adjusted to 7.2 to simulate biological conditions.

Fifty mL of a concentrated, well-dispersed aqueous alumina-comb polymer suspension (33.3% solids by volume, 1 mg comb polymer/m2 of alumina) is prepared by the following procedure. First, 0.70 g of the comb polymer and 24.3 g of deionized water are combined and mixed for 10 min to form a solution. Then, the solution is combined with 66.167 g of the as-received alumina precursor (AKP 50, Sumitomo). The resultant mixture is ultrasonicated with 1 sec on/off pulses for 5 min, followed by 12 h of stirring to form a dispersed, colloidal alumina suspension. A pH of 9 is maintained throughout the suspension preparation procedure.

About nine grams of an acrylic latex binder (16.7% solids by volume) is added to the aqueous alumina-comb polymer suspension and mixed for 2 h. The ceramic substrate is then formed with a tape casting procedure, as described by Reed, pp. 525–541 in Principles of Ceramic Processing, 2nd ed, John Wiley & Sons, Inc. (1995), incorporated by reference in its entirety, except that in the event of any inconsistent disclosure or definition from the present application, the disclosure or definition herein shall be deemed to prevail.

After casting, the tape is sectioned to the appropriate dimensions and dried under controlled-humidity conditions. The binder is removed by heating the green ceramic substrate at 10° C./min to 250° C., 1° C./min to 300° C., 1 h dwell at 300° C., 1° C./min to 600° C., and finally, a 1 h dwell at 600° C. The substrate is then sintered in air at 1600° C. for 1 h.

As any person skilled in the art of colloidal suspensions will recognize from the previous description, figures, and examples that modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of the invention defined by the following claims.